The invention is generally in the field of ink jet print heads. In particular, the invention relates to ink jet print heads suitable for use with pigment based inks.
Ink jet printing using dye based or hot melt inks is widely used for certain types of print runs, in particular, relatively short print runs of relatively large poster size prints. However, such inks are not suitable for billboard posters or other outdoor applications as they tend to fade rapidly.
Whilst the color fastness of pigment based inks is greater than that of dye based or hot melt inks, they have been found to rapidly clog ink jet nozzles of a conventional ink jet print head, thereby causing tell-tale white lines on a print of reduced quality.
Consequently, whilst desirable in terms of their color fastness, pigment based inks are not commonly used in ink jet printing applications to avoid the time consuming and expensive downtime required for cleaning purposes to ensure the printing of high quality prints.
The primary object of the present invention is to provide an ink jet print head suitable for use with pigment based inks.
A further object of the present invention is to adapt a conventional ink jet print head constituted by a composite body having parts of dissimilar metal hitherto adapted for use with dye based or hot melt inks so as to be suitable for use with pigment based inks.
In accordance with the teachings of the present invention, there is provided an ink jet print head suitable for use with pigment based inks, the ink jet print head comprising:
a composite body formed with an array of ink jet nozzles for respectively emitting a train of ink droplets, each of said ink jet nozzles being associated with a piezoelectric transducer and adapted for flow communication with a source of pigment based ink via an ink jet duct;
said composite body constituted by a plurality of constituent parts disposed substantially transverse to the direction of emission of a train of ink droplets, said composite body being made from two or more dissimilar metal parts of different electrochemical potential and thereby having one or more pairs of juxtaposed dissimilar metal surfaces each having a relatively large electrochemical potential difference thereacross;
at least one pair of juxtaposed dissimilar metal surfaces having a relatively substantial electrochemical potential thereacross have a protective layer interdisposed therebetween to at least substantially reduce said potential difference,
characterized in that
said composite body includes one or more aluminum parts each having a surface juxtaposed against a dissimilar metal surface, said aluminum parts having at least said surfaces coated, thereby reducing clogging of the ink jet nozzles of the ink jet print head when used with pigment based inks.
As shown in FIGS. 1 and 2, a conventional ink jet print head 1 for use with dye based or hot melt inks includes an aluminum housing 2 provided with an array of piezoelectric transducers 3, three stainless steel separator plates 4, 5 and 7, an aluminum chamber plate 8 and a nickel orifice plate 9 bonded together by adhesive to form a single sandwiched composite body 10 (see FIG. 2).
As shown, the ink jet print head 1 has an array of ink jet nozzles 11 each associated with a piezoelectric transducer 3 and in flow communication with an ink channel 13 via an ink jet duct 14, the ink channel 13 substantially extending along the length of the chamber plate 8 and adapted to be in flow communication with a source of pigment based ink (not shown).
Experimental work has shown that the hitherto occurring clogging of the ink jet nozzles of a conventional ink jet print head when used with pigment based inks can be substantially reduced by the provision of a protective layer between each pair of juxtaposed dissimilar metal surfaces.
On the basis of this experimental work, whilst the mechanism of the hitherto occurring clogging is not fully understood, it is believed that each pair of juxtaposed dissimilar metal surfaces in the composite body acts each pair juxtaposed dissimilar metal surfaces in the composite body acts as the plates of a Galvanic cell having a Galvanic current flow which facilitates the agglomeration of pigment particles in a pigment based ink.
It is known that aluminum, stainless steel and nickel have the following respective electrochemical potentials: xe2x88x920.75V, xe2x88x920.35V and xe2x88x920.15V, and consequently, the composite body includes three Galvanic cells as follows:
(i) a Galvanic cell between the aluminum housing""s front surface 2A and the stainless steel trailing separator plate""s rear surface 4A, the Galvanic cell consequently having an electrochemical potential difference of 0.4V;
(ii) a Galvanic cell between the stainless steel leading separator plate""s front surface 7A and the aluminum chamber plate""s rear surface 8A, the Galvanic cell consequently having an electrochemical potential difference of 0.4V; and
(iii) a Galvanic cell between the aluminum chamber plate""s front surface 8B and the nickel orifice plate""s rear surface 9A, the Galvanic cell consequently having an electrochemical potential difference of 0.6V.
In practice, the results of experimental work have indicated that a protective layer is required to preferably suppress a relatively large electrochemical potential difference across a pair of juxtaposed dissimilar metal surfaces or at least substantially reduce it to about 0.2V.